Adoptive immunotherapy, which involves the transfer of autologous antigen-specific T cells generated ex vivo, is a promising strategy to treat viral infections and cancer. The T cells used for adoptive immunotherapy can be generated either by expansion of antigen-specific T cells or redirection of T cells through genetic engineering (Park, Rosenberg et al. 2011). Transfer of viral antigen specific T cells is a well-established procedure used for the treatment of transplant associated viral infections and rare viral-related malignancies. Similarly, isolation and transfer of tumor specific T cells has been shown to be successful in treating melanoma.
Novel specificities in T cells have been successfully generated through the genetic transfer of transgenic T cell receptors or chimeric antigen receptors (CARs) (Jena, Dotti et al. 2010). CARs are synthetic receptors consisting of a targeting moiety that is associated with one or more signaling domains in a single fusion molecule. In general, the binding moiety of a CAR consists of an antigen-binding domain of a single-chain antibody (scFv), comprising the light and variable fragments of a monoclonal antibody joined by a flexible linker. Binding moieties based on receptor or ligand domains have also been used successfully. The signaling domains for first generation CARs are derived from the cytoplasmic region of the CD3zeta or the Fc receptor gamma chains. First generation CARs have been shown to successfully redirect T cell cytotoxicity, however, they failed to provide prolonged expansion and anti-tumor activity in vivo. Signaling domains from co-stimulatory molecules including CD28, OX-40 (CD134), and 4-1BB (CD137) have been added alone (second generation) or in combination (third generation) to enhance survival and increase proliferation of CAR modified T cells. CARs have successfully allowed T cells to be redirected against antigens expressed at the surface of tumor cells from various malignancies including lymphomas and solid tumors (Jena, Dotti et al. 2010).
Present CAR architectures are built on a design in which all relevant domains are contained within a single polypeptide (U.S. Pat. No. 7,741,465). This design necessitates serial appending of signaling domains, thus necessitating moving some domains from their natural juxtamembrane positions, distal from the plasma membrane. Nevertheless, architectures in which ligands and signaling domains are separate in their normal juxtamembrane positions (i.e. adjacent to the cell membrane on the internal side of it) would be more desirable and deemed to allow improved function of costimulatory domains. The high affinity receptor for IgE (FcεRI), could afford such architecture. Indeed, FcεRI, which is present on mast cells and basophils, is a tetrameric complex consisting of a ligand binding alpha subunit, a beta subunit and a homodimer of two signal-transducing gamma subunits (Metzger, Alcaraz et al. 1986). FcεRI alpha domain consists of an extracellular domain containing two Ig-like domains that bind IgE, a transmembrane domain and a short cytoplasmic tail. Beta subunit contains four transmembrane segments separating amino and carboxy terminal cytoplasmic tails. The gamma chain consists essentially of a transmembrane region and cytoplasmic tail containing one immunoreceptor tyrosine-based activation motif (ITAM) (Cambier 1995).
The current protocol for treatment of patients using adoptive immunotherapy is based on autologous cell transfer. In this approach, T lymphocytes are recovered from patients, genetically modified or selected ex vivo, cultivated in vitro in order to amplify the number of cells if necessary and finally infused into the patient. In addition to lymphocyte infusion, the host may be manipulated in other ways that support the engraftment of the T cells or their participation in an immune response, for example pre-conditioning (with radiation or chemotherapy) and administration of lymphocyte growth factors (such as IL-2). Each patient receives an individually fabricated treatment, using the patient's own lymphocytes (i.e. an autologous therapy).
Autologous therapies face substantial technical and logistic hurdles to practical application, their generation requires expensive dedicated facilities and expert personnel, they must be generated in a short time following a patient's diagnosis, and in many cases, pretreatment of the patient has resulted in degraded immune function, such that the patient's lymphocytes may be poorly functional and present in very low numbers. Because of these hurdles, each patient's autologous cell preparation is effectively a new product, resulting in substantial variations in efficacy and safety. Ideally, one would like to use a standardized therapy in which allogeneic therapeutic cells could be pre-manufactured, characterized in detail, and available for immediate administration to patients. By allogeneic it is meant that the cells are obtained from individuals belonging to the same species but are genetically dissimilar. However, the use of allogeneic cells presently has many drawbacks. In immune-competent hosts allogeneic cells are rapidly rejected, a process termed host versus graft rejection (HvG), and this substantially limits the efficacy of the transferred cells. In immune-incompetent hosts, allogeneic cells are able to engraft, but their endogenous TCR specificities recognize the host tissue as foreign, resulting in graft versus host disease (GvHD), which can lead to serious tissue damage and death. In order to effectively use allogeneic cells, both of these problems must be overcome.
In immunocompetent hosts, allogeneic cells are rapidly rejected by the host immune system. It has been demonstrated that, allogeneic leukocytes present in non-irradiated blood products will persist for no more than 5 to 6 days. (Boni, Muranski et al. 2008). Thus, to prevent rejection of allogeneic cells, the host's immune system must be effectively suppressed. Glucocorticoidsteroids are widely used therapeutically for immunosuppression (Coutinho and Chapman 2011). This class of steroid hormones binds to the glucocorticoid receptor (GR) present in the cytosol of T cells resulting in the translocation into the nucleus and the binding of specific DNA motifs that regulate the expression of a number of genes involved in the immunologic process. Treatment of T cells with glucocorticoid steroids results in reduced levels of cytokine production leading to T cell anergy and interfering in T cell activation. Alemtuzumab, also known as CAMPATH1-H, is a humanized monoclonal antibody targeting CD52, a 12 amino acid glycosylphosphatidyl-inositol- (GPI) linked glycoprotein (Waldmann and Hale 2005). CD52 is expressed at high levels on T and B lymphocytes and lower levels on monocytes while being absent on granulocytes and bone marrow precursors. Treatment with Alemtuzumab, a humanized monoclonal antibody directed against CD52, has been shown to induce a rapid depletion of circulating lymphocytes and monocytes. It is frequently used in the treatment of T cell lymphomas and in certain cases as part of a conditioning regimen for transplantation. However, in the case of adoptive immunotherapy the use of immunosuppressive drugs will also have a detrimental effect on the introduced therapeutic T cells. Therefore, to effectively use an adoptive immunotherapy approach in these conditions, the introduced cells would need to be resistant to the immunosuppressive treatment.
On the other hand, T cell receptors (TCR) are cell surface receptors that participate in the activation of T cells in response to the presentation of antigen. The TCR is generally made from two chains, alpha and beta, which assemble to form a heterodimer and associates with the CD3-transducing subunits to form the T-cell receptor complex present on the cell surface. Each alpha and beta chain of the TCR consists of an immunoglobulin-like N-terminal variable (V) and constant (C) region, a hydrophobic transmembrane domain, and a short cytoplasmic region. As for immunoglobulin molecules, the variable region of the alpha and beta chains are generated by V(D)J recombination, creating a large diversity of antigen specificities within the population of T cells. However, in contrast to immunoglobulins that recognize intact antigen, T cells are activated by processed peptide fragments in association with an MHC molecule, introducing an extra dimension to antigen recognition by T cells, known as MHC restriction. Recognition of MHC disparities between the donor and recipient through the T cell receptor leads to T cell proliferation and the potential development of GVHD. It has been shown that normal surface expression of the TCR depends on the coordinated synthesis and assembly of all seven components of the complex (Ashwell and Klusner 1990). The inactivation of TCRalpha or TCRbeta can result in the elimination of the TCR from the surface of T cells preventing recognition of alloantigen and thus GVHD. However, TCR disruption results in the elimination of the CD3 signaling component and alters the means of further T cell expansion.
T-cell mediated immunity includes multiple sequential steps regulated by a balance between co-stimulatory and inhibitory signals that fine-tune the immunity response. The inhibitory signals referred to as immune checkpoints are crucial for the maintenance of self-tolerance and also to limit immune-mediated collateral tissue damage. The expression of immune checkpoints protein can be deregulated by tumours. The ability of tumours to co-opt these inhibitory pathways represents an important mechanism in immune resistance and limits the success of immunotherapy. One of promising approaches to activating therapeutic T-cell immune response is the blockade of these immune checkpoints (Pardoll 2012). Immune checkpoints represent significant barriers to activation of functional cellular immunity in cancer, and antagonistic antibodies specific for inhibitory ligands on T cells including CTLA4 and programmed death-1 (PD-1) are examples of targeted agents being evaluated in the clinics.
Cytotoxic-T-lymphocyte-associated antigen 4 (CTLA-4; also known as CD152) down-regulates the amplitude of T cell activation and treatment with antagonist CTLA4 antibodies (ipilimumab) has shown a survival benefit in patients with melanoma (Robert and Mateus 2011). Programmed cell death protein 1 (PD1 or PDCD1 also known as CD279) represent another very promising target for immunotherapy (Pardoll and Drake 2012; Pardoll 2012). In contrast to CTLA-4, PD1 limits T cell effector functions in peripheral tissue at the time of an inflammatory response to infection and to limit autoimmunity. The first clinical trial with PD1 antibody shows some cases of tumour regression (Brahmer, Drake et al. 2010). Multiple additional immune checkpoint protein represent promising targets for therapeutic blockade based on recently studies.
In normal T-cells, T cell receptors emanate from the pre-T cell receptors (pTCR) which are expressed by immature thymocytes and are crucial for T cell development from the double negative (CD4− CD8−) to the double-positive (CD4+ CD8+) stages. Pre-T cells that succeed in productive rearrangements of the TCRbeta locus express a functional TCRbeta chain which pairs with an invariant preTalpha chain and CD3 signaling components to form the pre-TCR complex. The expression of the preTCR at the cell surface is necessary for triggering beta-selection, a process that induces the expansion of developing T cells, enforces allelic exclusion of the TCRbeta locus and results in the induction of rearrangements at the TCRalpha locus (von Boehmer 2005). After productive TCRalpha rearrangements and substitution of pTalpha by TCRalpha to form a mature TCR, thymocytes undergo a second step of selection, referred to as positive or TCRalpha/beta selection upon binding of self peptide MHC complexes expressed on thymic epithelial cells. Thus, mature T cells recognize and respond to the antigen/MHC complex through their TCR. The most immediate consequence of TCR activation is the initiation of signaling pathways via the associated CD3 subunits that result in multiple events including clonal expansion of T cells, upregulation of activation markers on the cell surface and induction of cytotoxicity or cytokine secretion.
Because of the nature of selection of TCRbeta chains through pairing with preTalpha during thymic development, in T cells in which TCRalpha has been inactivated, the heterologous introduction of the pTalpha transgene can result in the formation of a preTCR. This pTCR can serve as a means of T cell activation or stimulation in a manner that is non-MHC dependent, thus for example allowing continued expansion of alpha/beta T-cells following TCRalpha inactivation. Importantly, the pTCR complex displays a similar biochemical composition as the TCR in terms of associated CD3 subunits (Carrasco, Ramiro et al. 2001). In addition, in contrast to the TCR, pre-TCR signaling may occur in part by a ligand independent event. The crystal structure of the pTCR extracellular domain has provided a structural basis for the possible ligand-independence of pTCR signaling. The pTCR has been shown to form a head to tail dimer where two pTalpha-TCRbeta heterodimers associate (Pang, Berry et al. 2010).
In the context of developing therapeutic grade engineered immune cells that can target malignant or infected cells, the inventors have sought for improved CAR architectures, which would be closer to natural ones and likely to behave accordingly using any extracellular mono or multi-specific ligand binding domains.
As a result, they have designed a new generation of CARs involving separate polypeptide sub-units according to the present invention, referred to as “multi-chain CARs”.